(i) Field of the Invention
The present invention relates to a process for galvanizing a metal component (strip, plate, etc.) in a continuous galvanizing line, the galvanizing line comprising, placed in series and connected to each other by ducts in order to form ducting for circulating a reducing atmosphere usually composed essentially of an inert gas, such as nitrogen or argon, and, of hydrogen, a preheat furnace, an annealing furnace, a cooling station and a station for dipping the metal component into the bath of liquid zinc or of a zinc alloy, in which process, before the metal component is dipped into the liquid bath, it is exposed to this reducing atmosphere in order to remove oxides present on the surface of the metal components.
The description which follows will refer to metal "strip" in order to be specific, and will refer indiscriminately to a bath of liquid zinc or a bath of liquid zinc alloy, without the reference chosen being regarded as restrictive, because, as is known, the industry uses alloys which are extremely varied, especially in their zinc and/or aluminum content.
In general, then, a continuous galvanizing line comprises at least four zones for treating the metal strip to be galvanized, namely a preheat zone, an annealing zone, a cooling zone and a dipping zone which comprises a zinc bath into which the metal strip to be galvanized is dipped.
(ii) Description of the Related Art
Galvanizing lines are known in which the preheat zone comprises a furnace fitted with naked-flame burners serving, on the one hand, to rapidly reheat the metal strip to be treated to a temperature typically of between 400.degree. C. and 700.degree. C. and, on the other hand, to make the rolling oils present on the surface of the strip undergo pyrolysis.
In order to prevent oxidation of the metal strip thus treated, the burners are operated in air depletion mode in order to provide an atmosphere which is nonoxidizing with respect to iron.
In order to be able to ensure good galvanizing, that is to say, in particular, good adhesion between coating and metal strip, it is absolutely essential to remove any surface oxide layer before the metal strip is dipped into the zinc bath. This is achieved by exposing the metal strip in the annealing furnace to a reducing atmosphere usually consisting of a mixture of nitrogen and hydrogen, the hydrogen content generally being between 15% and 40%.
For this purpose, the various treatment zones of the galvanizing line are connected together by ducts in order to form ducting for circulating the reducing atmosphere.
In order to constantly regenerate this reducing atmosphere in this ducting and thus to preserve its reducing nature, the mixture of nitrogen and hydrogen is injected into a duct also called a spout or nozzle, one end of which dips into the zinc bath and the other end is joined to the outlet end of the cooling station, so that the reducing atmosphere flows in the opposite direction to the direction in which the metal strip to be treated runs.
At the present time, for a given galvanizing line, the flow rate of the mixture of nitrogen and hydrogen and the hydrogen content of this mixture are maintained at the same level, independently of the characteristics and the run speed of the metal strip to be treated.
In practice, in order to make it possible both to treat very wide metal strips and narrow strips and to accommodate low run speeds and high speeds, the flow rate of the mixture of nitrogen and hydrogen and the hydrogen content of the mixture are fixed at a high level so as to allow the treatment even of the most unfavorable cases, i.e. metal strip of large surface dimensions and/or treated at high speeds. However, it may be imagined that this excessive quality represented by a hydrogen-rich mixture injected at a high rate entails a not insignificant cost for this reducing atmosphere. Moreover, since the atmosphere injection conditions are fixed, which the surface to be treated per unit time may vary, the production of water vapor in the enclosure, because of the reduction of the oxides, will well and truly make the reducing nature of the atmosphere vary and therefore cause variations in the quality of the final product.